The old version you have is part of the old suite dubbed "RadControls for ASP Classic" which has not been supported since 2009. The current suite, "UI for ASP.NET AJAX", while based on them, is entirely new in terms of plaforms and concepts and thus, there are a lot of changes. We do try to keep backwards compatibility but sometimes changes are needed. They are inevitable when we changed platforms 7 years ago. We do try to keep the current suite in a stable state with as little changes as possible.
Some of the old skins are not available anymore. You can find the full list in the online demos where a skin chooser is available or here: http://www.telerik.com/help/aspnet-ajax/introduction-how-skins-work.html. You can take a look at the demos to see which of the current ones are closest to your needs to switch to them. You can also set the skin globally from the web.config for all controls: http://www.telerik.com/help/aspnet-ajax/introduction-skin-registration.html (web.config registration is shown at the end).
Custom skins can be used with the current suite and this article explains the basics: http://www.telerik.com/help/aspnet-ajax/introduction-create-custom-skin.html. Each control has an article that explains details, for example: http://www.telerik.com/help/aspnet-ajax/tooltip-appearance-creating-custom-skin.html. This article also explains that you need to set the EnableEmbeddedSkins to false to use custom skins. How exactly you are going to set this property and the Skin property of the control depends on your site and preferences, there is no silver bullet for this choice.
There has been a major change in Q1 2009 to unify the skinning approach in all controls and this causes old custom skins to have some issues. Thus, I advise that you go through the following resources to resolve that:
I hope this helps you migrate the customizations you have.
I would also like to use this opportunity to strongly advise that you upgrade as often as possible (e.g., with every service pack release). This minimizes changes that you may need to tackle and also provides you with the latest fixes, improvements and new features/controls.
Generally, when upgrading I would suggest having those resources in mind:
You may also find this blog post useful when upgrading:
Also, in case you are using the "Classic" suite of controls, you may find articles like this one useful:
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